Safety belts with buckles of the type indicated in the introduction to claim 1 are described, e.g. in the Swedish Patent Application No. 7704854-4. This known mechanism includes a detent member movable in the direction of entry of a locking tongue forming part of the device while a pivoting locking member is constructed in such a way that the locking member may be pivoted to a release position when the detent member is moved in the direction mentioned. The locking member then presents an edge which must be passed, there then being a risk of the detent member catching on this edge leading to partial locking. Partial locking is a very difficult problem to overcome when designing safety belt buckles, and is a very essential problem as partial locking may mean that the two interconnecting buckle parts are separated under loads on the safety belt, e.g. in an accident.